1. Field of the Invention
The present invention relates to a method and an apparatus for measuring electrical signals in integrated circuits by utilizing the electro-optic effect and laser beams.
2. Description of the Background Art
When an electro-optic material which changes its index of refraction according to the surrounding electric field is placed inside the electric field produced by an operating integrated circuit while laser beams propagate through this electro-optic material, the polarization of the laser beams changes due to the change of the index of refraction of the electro-optic material. This change of the polarization of the laser beams can be converted into the change of the beam intensity by means of an optical system for polarization detection using a polarizer. Thus, by measuring the change of the beam intensity of the laser beams illuminating the electro-optic material, it is possible to measure the electric field coupled to the electro-optic material, i.e., the voltage signal at a point of measurement at which the electro-optic material is located. This method of measuring the voltage signal of the integrated circuit has been known as the electro-optic sampling.
In such an electro-optic sampling, the coupling of the electric field to the electro-optic probe using the electro-optic material varies in correspondence to a distance between the probe and the circuit under examination (referred hereafter as a gap), so that the detected change of the beam intensity strongly depends on the gap with which the measurement was made.
However, conventionally, there has been no established procedure for the positioning of the probe to determine the gap precisely, and the measurement has been made either by bringing the probe as close to the circuit under examination as the sufficient S/N ratio for making the measurement of the voltage signal can be realized, or by bringing the probe in contact with the circuit under examination.
For this reason, the reproducibility of the measurement has been poor because of the variation of the gaps used in different measurement trials, and while it has been possible to obtain the timing information such as a relative timing delay and rise time and fall time of the waveform, it has not been possible to obtain the information on the waveform amplitude and the absolute voltage which is particularly valuable in the measurement of the analog circuit for the purpose of facilitating the comparison of the waveforms.